My '99 A4 Jetta has been hard to start when hot since I bought the car. This is my third VW diesel and I thought that this car would operate like the others. I was used to putting the key in the ignition and b-a-r-e-l-y turning the key to have the 'beast' start. The timing is good, nothing sticking with the crossover valve, no fuel leaks, everything looks as it should. Recently, I thought that the coolant sensor was to blame. I ordered a new one, and while waiting for delivery I disconnected the plug. The engine started up right away, and with great vigor, very similar to the way the two older models would start. I put the new coolant sensor in, and back to difficult starting.

Question for all:: are there different software versions for the different model years??
My feeling is that this is a software issue since the car started better with the sensor disconnected??
Any and all thoughts.

I'm working on a 99 (A-4) Golf with this very same problem.
The timing is set slightly above the mid-line on the timing graph.
Its not the starter because once the eng. temp sensor (ETS) is disconnected the engine starts flawlessly.

So far I've swapped (with known good units) the electrical load reduction relay and the GP relays. This proved no change in the problem.
I've already replaced the ETC. The GP all check good (.4 to .7) Ohms.

Here is where it gets interesting. With the ETS connected I am only getting 0.13 VDC to the GPs.

When the engine is dead cold....this engine starts just fine. This problem (warm start) happens ONLY when the engine is at normal operating temp (190*F). Shut off the engine ...wait 5 minutes and it takes excessive cranking to get the engine started. Excessive cranking is defined as 5+> seconds of constantant cranking.

This tells me that the circut wiring is not damaged. All of the GP circut components are working. The only thing left is the ECU. But I'm really hesitent condeming this component untill further trouble shooting and diagnostics have been done.

My car does the same thing.. Starts up quick when cold, when up to operating temps, 5+ seconds before it starts up. So when you unplug the ETS, when the car is at operating temp it starts up right away?

My car does the same thing.. Starts up quick when cold, when up to operating temps, 5+ seconds before it starts up. So when you unplug the ETS, when the car is at operating temp it starts up right away?

My car does the same thing.. Starts up quick when cold, when up to operating temps, 5+ seconds before it starts up. So when you unplug the ETS, when the car is at operating temp it starts up right away?

I am having the same issue. 1999.5 - starter replaced - glow plugs replaced - new battery - new MAF - this is really annoying. I bought the car with 100k miles on it and it started like that ever since. It now has 170k miles on it. The car starts fine when cold (not like my father's 2003 did when it was new though). I run it for a while (until its up to 190F). Let it sit and then it takes excessive cranks to start (5+ or so). I tried disconnecting the temp sensor and this seemed to help but there is nothing definitive.

My father's 2003 (90k miles) is having an even worse problem, his car starts fine in the cold but when it is warmer out it starts hard, he says it just keep turning over and a light comes on (not sure what light). He stops and trys again. On the 3rd try it starts. Its awful. Seems like a different issue though.

Timing is right within bands also. On mine it seems to start easier if I don't wait for the glowplugs and just start cranking.

I posted this reply before. I have a 99.5 with the same issue. It is not a timing problem. My timing is set at the top of the graph (advanced) and it doesn't make a difference. New battery, nothing. It's not the starter either, or the anti-shudder valve. No bubbles in the fuel line. It has to be something else.

My understanding of how the software operates (someone correct me if I am wrong) is that when cold, the glow plugs operate and fuel injection begins immediately while cranking. When hot, there is no glow plug operation and fuel injection is delayed until the engine reaches a certain cranking speed (100rpm?).

Is it possible that the crank speed sensor might not be sending a good signal at such a low RPM? Maybe it is a bit out of spec or something. Would be interesting to swap one out with a new sensor or somehow trick the CPU (by sending a generated signal) and see if that makes a difference.

...Is it possible that the crank speed sensor might not be sending a good signal at such a low RPM? Maybe it is a bit out of spec or something. Would be interesting to swap one out with a new sensor or somehow trick the CPU (by sending a generated signal) and see if that makes a difference.

The engine cranking speed (250 RPM) you are refering to is the minimum "recommended" cranking speed to achive diesel ignition.
This is with a fully charge battery with at least 650 CCA @ 32*F.
However....even with a "weak" battery the engine will eventually still start but hesitently (excessive smoke ect).

However...it wouldn't be that difficault to test your theory by swapping out the engine speed sensor (ESS). The only issue is coming up with a spare ESS.

I'm rather lucky having a full set of A-4 TDI spare relays (that I salvaged from my Golf that was totaled). I use these realys to test & diagnose such problems as this issue.

IMHO it could be slightly clogged intake manifold. Engine doesn't get a good gulp of air to compress = low cylinder heat = slow start. Add glow plug and get extra heat in cylinder = quick starts. If you preglow and it starts right up forget the starter and battery as you have just place an extra load on the battery.

IMHO it could be slightly clogged intake manifold. Engine doesn't get a good gulp of air to compress = low cylinder heat = slow start. Add glow plug and get extra heat in cylinder = quick starts. If you preglow and it starts right up forget the starter and battery as you have just place an extra load on the battery.

Not sure who this reply is to but both the vehicles I am discussing had there intake manifolds cleaned recently and were heavily clogged. The problem is that they're starting hard when warm, not when cold. What you're saying is opposite to the issue.

tdi_greendale I too thought like you when I had the theory mentioned to me the first time. It is the high temperture of the air in the cylinder that ignites the fuel charge caused by air being highly compessed. That's how compession ignition works. If you can not get a good amount for air in the cylinder the temperture of the air is lower causing a no fire in that cylinder. With the use of glow plugs the cylinders internal air temperture goes up causing a plugged intake manifold engine to start. I guess I didn't understand that when the motor is up to operating temperture that the use of glow plugs helped the situation. That's what I read so that's what prompted my response. Just trying to help. Timing set in the middle of the upper portion of the timing graf will definatly help with slow starting (long crank time) in tempertures that do not require glow plug operation.

This is the exact problem that I am having as well. I have read all the threads regarding timing, mine is in the upper half of the chart on VAG-COM. Nothing has helped.

I have avoided the temptation to wire a switch into the ETS circuit to fool the ECU into thinking it is uplugged when starting warm.

Nothing I have done fixes this. The only change I have experienced is with a change in injector nozzles. The old stock nozzles started a little quicker than the new ones (Sprint 520). I am at 220,000 on my Jetta

how r u doing with that hard start Herm? I too have one of these "slow to start when warm" buggers here in the shop and have done all that has been discussed here (and other threads) but when warm, she'll crank 4-7 seconds. fire up, no smoke, will run great. This seems to be a relatively common problem. Perplexing.

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(1)2002 jetta tdi-gls-5sp-204k-pp520's--(2)2002 jetta tdi GL-5sp-PP520-200K--(3)2003 jetta tdi 5pd, broken but will fixA! Got a real job but still fixing on week ends and some nights-phone me if needed-609-713-1414--Just moved and can resume working on tdi's...presently only working on ALH cars.

Add me to the list. I'm going to set up a session with Herm, but I've done the same as him, and gotten the same results. I've checked the GP harness the same and got the same results! I'm also throwing a CEL code which we will check out also.